1. Field of the Invention
The present invention relates to an exhaust purifying device applicable to the exhaust system of an internal combustion engine, e.g., an automobile internal combustion engine and, more particularly, to an exhaust purifying device of the type including a core provided with a honeycomb structure and applied with a catalyst.
2. Description of the Related Art
An exhaust purifying device of the type described, i.e., a so-called catalytic converter has customarily been designed to communicate to the exhaust system of, for example, an automobile for promoting the oxidation of exhaust gases emanating from the exhaust system. Specifically, the catalytic converter is associated with an exhaust pipe extending from an engine or a muffler and applied with a catalyst for reducing the concentration s of carbon monoxide, hydrocarbons and other toxic gases contained in the exhaust. One type of conventional catalytic converter has a hollow cylindrical casing and a metallic core received in the casing and is provided with a honeycomb structure. A catalyst is applied to the metallic core. The casing is affixed to a predetermined portion of the exhaust pipe as the muffler of the exhaust system. Specifically, the core having a honeycomb structure is implemented by a corrugated sheet of stainless steel or similar metal, and a flat sheet also made of metal. The corrugated sheet and flat sheet are superposed on each other and rolled up together in a spiral configuration. The core is inserted into and affixed to the inner periphery of the cylindrical casing by a solder. A nickel powder is one of the solders available for such an application. To affix the core to the casing, three different methods are available, including one which mixes a binder with a solder to prepare a paste and applies it to the outer periphery of the core (referred to as a first method hereinafter), one which applies an adhesive to the outer periphery of the core beforehand and then deposits a solder on the adhesive (referred to as a second method hereinafter), and one which wraps an amorphous solder in the form of foil around the core (referred to as a third method hereinafter).
The first to third conventional methods described above each has some problems left unsolved, as follows. The first method has a problem that when the core is inserted into the casing, the solder on the outer periphery of the core is sequentially forced out from one end of the casing due to the friction acting between the core and the casing. This makes it difficult to supply a required amount of solder as far as a predetermined position. The problem with the second method is that it is difficult to supply the solder between the core and the casing since a gap is not available therebetween. Therefore, the connection relying on the first method or the second method fails to firmly affix the core and casing. Assume that such a defective catalytic converter is mounted on the muffler or the exhaust pipe. Then, it is likely that the core is separated from the casing due to, for example, the vibration of the vehicle body and the pressure of the exhaust and thereby moved within the casing, causing the catalyst to come off from the surface of the core.
The third method is superior to the first method since it distributes the solder uniformly over the entire outer periphery of the core without the intermediary of a binder. However, the foil of amorphous solder is expensive and has to be wound by an extra and complicated step. Moreover, the foil is wound around the entire outer periphery of the core and affixed to the entire inner periphery of the casing. This brings about another problem that when the core and the casing each expands and contracts in a particular manner due to the high-temperature and high-pressure exhaust, the interior of the core is likely to sequentially deform since the difference in expansion and contraction cannot be absorbed. Then, the core is rapidly deteriorated, i.e., the honeycomb structure is rapidly distorted to make the passage of the exhaust irregular. This prevents the catalyst from exhibiting the expected function.